"We have not met, but you are now part of our extended family," writes Barden. "It is not a family we chose, but a family born from the horrible circumstance of losing a child to gun violence – one that's growing steadily."

Barden posted his letter to Martinez from the nonprofit Sandy Hook Promise's group page on Facebook.

"My heart breaks for you because I know just a little about the long road ahead of you. We have reached out to you privately but publicly we wanted to say to you and those feeling the sorrow, anger and frustration of this weeks' shooting, you are not alone," he wrote.

Martinez, whose son Chris was one of seven who died, has been speaking out about gun violence since the tragedy.